Book/Report

FZJ-2018-02691

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Eingefrorene Orientierungsunordnung und Rotationsanregungen in festen Mischungen von Methan und Krypton (Neutronenstreuexperimente)

Grondey, S. (Corresponding author)FZJ*

1986
Kernforschungsanlage Jülich, Verlag Jülich

Please use a persistent id in citations: http://hdl.handle.net/2128/18322

Report No.: Juel-2083

Abstract: The effect of a statistical replacement of CH 4 molecules by Kr atoms on the rotational states in solid methane has been examined. Obviously the anisotropic molecular interaction (octopole octopole interaction) is disturbed in a way analogous to magnetic systems with random internal fields. Inelastic neutron scattering experiments on solid mixtures (CH$_{4})_{1-x}Kr_{x}$ with 0 $\le$ 0.35 have been carried out, and simple models have been developed to interpret the spectra. Three regions of the phase diagram (x,T) have been examined to answer in particular the questions (a) How are the rotational excitations in phase II disturbed by the admixture of Kr (x$_{Kr} \le$ 0.20) ? (b) Which 'frozen-in orientational disorder' appears at higher Kr concentrations? (c) How can the molecular rotation at high temperatures be characterized in a simple way? In phase II (x$_{Kr} \le$ 0.20) contributions from rotational tunnelling transitions (from 3 of 4 molecules) and transitions of nearly free rotors (from 1 of 4 molecules) are present in the scattering intensity. For x$_{Kr} \le$ 0.1 these contributions are clearly separable and exhibit a characteristicbroadening and asymmetry. At these Kr concentrations the shape of the tunnelling lines could be quite well reproduced with a model based an a statistical distribution of different molecular surroundings; this model describes the rotational potentials and energies in a 'local mean field approximation'. The parameter determining the potential is the number of Kr atoms an ordered nearest neighbour sites of a tunnelling molecule. The measurements on samples with x$_{Kr}$ = 0.27 and 0.35 indicate disordered orientations at all temperatures. The disorder at high temperatures (i. e. at T $\gtrsim$ 30 K) is dynamical and can be best described as classical rotational diffusion with only weak hindering potentials. At low temperatures, on the other hand, there is static disorder and the 'dynamics' are restricted to quantum mechanical rotational transitions. A model with a statistical distribution of local potentials was used to fit the date ; the distribution depends an the probabilities for the molecules to be in their ground - or first excited states, and therefore on the degree of spin conversion in the samples. Orientational correlations, which were indicated by earlier single crystal measurements on a deuterated mixture, have been neglected in the model. The transition from dynamical to frozen-in orientational disorder proceeds continuously with decreasing temperature. Similar to other diluted molecular mixtures the System $(CH_{4})_{1-x}Kr_{x}$ with x$_{Kr} \gtrsim$ 0.20 can thus be characterized as a ' paramagnetic ' phase with orientational correlations which are frozen-in at low temperatures, but without Jong-range orientational order.

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Contributing Institute(s):

1. Publikationen vor 2000 (PRE-2000)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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